In computer storage, a logical unit (LUN) is an address for an individual disk drive and by extension, the disk device itself. The term is used in small computer systems interface (SCSI) protocol to differentiate individual disk drives within a common SCSI target device or disk array. LUN is a common term used in storage area networks (SAN) and other enterprise storage fields. LUN's are not usually used to describe individual disk drives, LUN more often refers to a virtual partition or volume of a RAID set.
Conventional enterprise storage subsystems are capable of storing many terabytes of data. Many users define a standard storage subsystem volume size and create many volumes of this size. Typically, the initial size is small, 8 GB, for example. As more storage is allocated the number of these small LUN's grows very quickly. For example, an IBM DS8000 with a capacity of 512 T byte could contain 48,000, 8G byte units. A very large number of storage subsystem volumes results in management complexity. Larger storage subsystem volumes may reduce management complexity to some extent. There is currently no way to resolve this problem, without an off-line conversion of the smaller volumes into larger ones, backup and restore of data from the server, and potential reconfiguration of all software and applications on the server. IBM's SAN Volume Controller (SVC) provides the ability to migrate storage extents (a multiple of blocks) between the “physical” storage subsystem disks it presents to the host operating system.
In SCSI, LUN's are addressed in conjunction with the controller ID of the host bus adapter, the target ID of the storage array, and an optional slice ID. In UNIX operating systems, such ID's are often combined, e.g., to refer to a controller, a target, a disk, and a slice, all in one word. EMC, Sun's Solaris operating system, Hewlett Packard's HP-UX, Hitachi Data Systems, NCR, and Teradata's MP-RAS use LUN slices, while IBM's AIX uses “hdiskN” to refer to a particular partition of a storage device.
Conventional storage volume consolidation processes require application downtime, and backup and restore of data. The prior art methods do not work while the system is on-line. Such prior art methods also require significant changes to the existing software or applications.